The present invention relates to a light source with a semiconductor junction, particularly a laser source and to a process for producing such a source. It is used in optics and more specifically in optical telecommunications.
Although the invention is applicable to any light source with a semiconductor junction, the following description refers in an explanatory manner to a particular category of such sources, namely coherent or laser sources. The reason is that such devices are of particular interest in the field of optical telecommunications. However, other sources (called light emitting diodes with a semiconductor "emitting by the edge") are not excluded from the scope of the invention. In an even more specific manner, the following description is particularly directed at lasers of the type having a double heterostructure and a strip junction, because such lasers constitute preferred sources in optical telecommunications using the near infrared. However, once again, other types of semiconductor lasers are not excluded from the invention. Finally, although the examples described will essentially relate to the InP semiconductor, the invention is in no way limited to this material. In fact, it covers all known materials fulfilling the conditions which will be defined hereinafter and particularly InP-based ternary or quaternary compounds or GaP-InP ternary compounds or GaAs-P-InGaP alloys.
It is known that a laser with a double heterostructure and a junction comprises a substrate on which are successively deposited a lower confinement layer, an active layer, an upper confinement layer and finally a contact layer.
In order to bring about a continuous operation at ambient temperature, the injection of charge carriers is confined to an active rectangular surface with a width of a few microns (10 to 20 .mu.m-strip) limited at the two ends by two cleavages.
The preferred field of such lasers consists of long distance optical fibre telecommunications for which the most interesting emission wavelengths are 1.3 and 1.55 .mu.m.
A laser using as the material GaInAsP/InP and operating continuously at ambient temperature is described in the article published by J. J. HSIEH in the Journal "Applied Physics Letters", Vol. 28, p. 709, 1976. This is a strip laser obtained by proton bombardment and emitting at 1.1 .mu.m. A laser emitting at 1.3 .mu.m is described in the article by K. OE et al published in the Journal "JapanJ.App.Phys", Vol. 16, 1977, No. 7, p. 1273. In this case, the strip is produced by means of ohmic contact deposited in the disengaged part of a dielectric (SiO.sub.2) covering the plate. An article entitled "GaInAsP/InP planar stripe lasers prepared by using sputtered SiO.sub.2 film as a Zn-diffusion mask" published by K. OE et al in the Journal "J.Appl. Phys", 51, (1), January 1980, p. 43 describes the influence of the technology used for producing the strip (SiO.sub.2 deposition and zinc diffusion) on the properties of the lasers obtained.
The article by R. J. Nelson et al entitled "High-output power in GaAsP (.lambda.=1.3 .mu.m) strip buried heterostructure lasers" published in the Journal "Appl. Phys.Lett" 36, (5), Mar. 1st 1980, p. 358 describes a strip (buried) laser using the compound InP.
The most frequently used strip definition method is that of placing a contact through a window defined in a dielectric layer covering the contact layer. This deposit may optionally serve as a diffusion mask, as described in the article of K. OE referred to hereinbefore. This article also shows the prejudicial influence of the constraints exerted by the dielectric on the zinc diffusion mechanisms beneath the strip and their effects on the mode structure of the laser. In addition, such constraints increase the speed at which the junctions deteriorate.
The methods of insulating the strip by proton bombardment have not as yet been performed in a satisfactory manner for lasers emitting at 1.3 .mu.m, doubtless due to an inadequate insulation of the bombarded regions.
The technology of lasers having a buried structure is too complicated (particularly due to epitaxy renewal) to obtain an adequate reproducibility and reliability.